This invention relates to mobile communications systems and particularly to means for determining handover and for power control adjustment in such a system.
One particular type of mobile communication system is a cellular radio telephone system. Cellular radio telephone systems generally include a switch controller coupled to the public switched telephone network (PSTN) and a plurality of base stations. One or more mobile stations communicate with a base station that facilitates a call between the mobile station and the PSTN. The communication link over a carrier signal from the base station to a mobile station is referred to as the downlink. Conversely, the communication link from a mobile station to the base station is referred to as the uplink.
Multiple access techniques permit the simultaneous transmissions from several mobile stations to a single base station over a plurality of communications channels. Some channels are used for carrying traffic e.g. voice, and others are used for transferring control information.
A particular type of cellular radio telephone system is the GSM (Global System for Mobile Communications) a description of which can be found in the book xe2x80x9cThe GSM System for Mobile Communicationsxe2x80x9d by M. Mouley and M. Pautet.
GSM networks generally include mobile services switching centres, base stations and mobile stations. Each of the plurality of base stations generally defines a geographic region or xe2x80x9ccellxe2x80x9d proximate to the base station to produce coverage areas.
One feature of the current GSM system allows the transceivers in the base station and mobile station to adjust their power output to take into account the distance between them. The closer the mobile station is to the base station""s transceiver, the less power it and the base station""s transceiver will be required to transmit. This feature saves battery power in the mobile station and also helps to reduce interference effects. Both uplink and downlink power settings can be controlled independently. Initial power settings for the subscriber unit along with other control information, is set by the information provided on a broadcast control channel (BCCH) for a particular cell. The base station controls the transmit power of both the mobile station and the base station""s transceiver. The received mobile station""s power is monitored by the base station and the power received from the base station""s transceiver at the mobile station is monitored by the mobile station and then reported to the base station. Using these measurements the power of both the mobile station and the base station""s transceiver can be adjusted accordingly.
As the mobile station moves from one cell to the next, the communication link is transferred from its current base station to a neighbouring base station using a procedure known as handover or handoff. The need for handover is usually determined on the basis of one or more criteria. One of the most commonly used criteria in a GSM system is the quality measure, the so-called RxQual, which is a measure of the bit error rate (BER). BER is calculated before de-interleaving and decoding processes, which are mechanisms to combat or correct errors in the received signal. Frame erasure rate (FER) is an indicator that measures the quality perceived by the subscriber and must be calculated after decoding and deinterleaving RxQual, i.e. BER, is the metric of quality, used to trigger the quality handovers and increase the power.
Frequency hopping is another GSM feature that allows a network operator to use a set of frequencies during a call instead of using only one. The use of a number of frequencies makes de-interleaving and decoding processes more robust and efficient against transmission errors because of the ability of frequency hopping to temporally spread the corrupted bits caused by interference and fading. In other words, signals received with the same BER result in a better FER for the hopping case. So, calls allocated on hopping time slots experience better final quality (perceived by the subscriber), than those on non-hopping channels.
The situation is that, for hopping, instead of the BER (RxQual), the better measure of the quality is the FER, measured after the decoding and de-interleaving processes. In currently employed systems, a unique quality threshold is used to trigger either the handover or the power control. I.e. the same handover criterion is used for both hopping and non-hopping communication channels.
The case most frequently found, is a system working with a fixed frequency plan and with the quality threshold optimised to the appropriate value according to the quality level tolerated before a handover or power increase is made. In this situation, if frequency hopping is implemented, some of the carriers (non-BCCH ones) are equipped with frequency hopping and some others (BCCH) are not. So both types of carriers co-exist in the same cell. The appropriate value for the threshold set for the non-hopping calls triggers handovers and power increases when necessary, whereas for hopping calls, they are triggered before the quality degradation requires it. Particularly, an increase in the number of handovers is noticed after frequency hopping implementation because a lot of unnecessary handovers are performed. If however, the quality thresholds are set to an appropriate value to trigger handovers and power adjustments when necessary for the hopping calls, cases of bad quality are noticed for non-hopping calls before reaching these thresholds.
According to a first aspect of the present invention, a system for determining handover in a mobile communications network comprises;
means for establishing communication between a mobile station and a base station over a communications channel,
means for determining whether the communications channel is a frequency hopping channel or a non-frequency channel,
means for monitoring a quality parameter of a signal received over the communications channel,
a first comparator for comparing the monitored quality parameter with a first threshold thereby generating a first trigger signal when the first threshold is exceeded,
a second comparator for comparing the monitored quality parameter with a second threshold, thereby generating a second trigger signal when the second threshold is exceeded,
and selector means, operably coupled to the determining means, for selecting the first trigger signal when the said channel is a non-frequency hopping channel, and the second trigger signal when said channel is a frequency hopping channel, thereby to trigger a handover.
The monitored quality parameter may, conveniently, be the BER of the received signal. The thresholds for the non-hopping and hopping cases are set to their optimum values by trial and error. This first aspect of the invention is applicable to instances where only a single quality measurement (eg BER) is available.
A second aspect of the invention is applicable when two distinct quality measurements are available.
According to a second aspect of the present invention a system for determining handover in a mobile communications network comprises;
means for establishing communication between a mobile station and a base station over a communications channel,
means for determining whether the communications channel is a frequency hopping channel or a non-frequency hopping channel,
means for monitoring a first quality parameter of a signal received over the communications channel,
means for monitoring a second quality parameter a signal received over the communications channel,
a first comparator for comparing the first quality parameter with a first threshold thereby generating a first trigger signal when the first threshold is exceeded,
a second comparator for comparing the second quality parameter with a second threshold, thereby generating a second trigger signal when the second threshold is exceeded,
and selector means, operably coupled to the determining means, for selecting the first trigger signal when the said channel is a non-frequency hopping channel, and the second trigger signal when said channel is a frequency hopping channel, thereby to trigger a handover.
The above principle of the present invention can also be applied to power control adjustment, with the output of the selector means being used to adjust the transmit power level depending upon whether the channel being used is a non-hopping or hopping one.
The first quality parameter may be the BER and the second quality parameter may be the FER of the received signal.
The system may be incorporated in the mobile station or the base station or both.
The advantages of using two different thresholds are achieved because of the different behaviour of the call according to perceived voice quality for the hopping and non-hopping cases. The improvements are as follows.
The two situations (hopping and non-hopping channels) are handled in the most appropriate way with both thresholds being set up separately to their optimum value, thus better equating to perceived quality.
The selection of a compromised threshold value, which is non-optimum for either situation, is avoided.
Bad quality situations arising on non-hopping channels before a handover or a power increase (i.e. when the threshold is optimised for a hopping channel) are eliminated. The overall voice quality on the network will increase.
Unnecessary handovers and unnecessary power increases (when the threshold is optimised for a non-hopping channel) are avoided. Therefore, there will be a reduction of signalling traffic and interference as well as an increase on battery life of the mobile station.
The reduction of handover failure rate leads favorably to a decrease in the dropped call rate. Furthermore, the decrease in the number of handovers per call whilst improving the performance, eliminates unnecessary handovers.